AN EXTRUDABLE CONDUCTING POLYMER COMPOUND

Abstract: An extrudable electrically conducting polymer composition, with improved strippability when used as outer conducting layer applied to a polyethylene insulation, in the form of a mixture comprising at least: 20 to 70 per cent by weight of a copolymer which consists of 20 to 80 percent by weight of an olefin and 80 to 20 per cent by weight of a polar monomer, such as vinyl acetate, ethylene acrylate, methylacrylate, butylacrylate, 5 to 60 per cent by weight of a modified polypropylene, 5 to 60 per cent by weight of an electrically conducting filler such as carbon black, to provide the desired electrical conductivity, and up to 5 per cent by weight of stabilizing additives such as antioxidants.

Description: A n extrudable conducting polymer compound TECHNICAL FIELD The invention relates to an extrudable conducting polymer composition and the use thereof as external conducting layer in an insulating system for electric cables. BACKGROUND ART More particularly, the invention relates to a conducting crosslinkable polymer composition which by means of extrusion is intended to be applied as a casing around an insulated electric conductor, where the insulating layer is a polymeric material in thermoplastic or cross-linked form. A layer or casing of a polymer composition according to the invention, extruded around the insulated conductor, exhibits improvements in the mechanical properties, above all in the temperature dependence of the mechanical properties, which permits the conducting layer or casing to be easily stripped off, during the mounting work, within a wide temperature interval while at the same time a good adhesion to the underlying insulation is retained. BACKGROUND ART For power cables where the conductor is arranged with an insulation of a thermoplastic or crosslinkable polymer, for example a polyethylene, it is often necessary to provide the insulation with an inner and an outer conducting layer to control the electric field strength. The conducting layers usually consist of extrudable electrically conducting polymer composition. Around the inner metallic conductor there is applied an inner conducting layer, the task of which is to equalize the elec¬ tric field between the conductor and the insulation. A second conducting layer is applied across the insulation. This outer conducting layer shall have sufficiently good pro¬ perties to function as an electric screen, and further this outer conducting layer must be easily removable from the inner insulation during joining and mounting work. In this way it is required that the outer conducting layer, on the one hand, shall have a sufficient adhesion to the insulation to ensure that the necessary electric screening is obtained and that no corona arises between the outer conducting layer and the insulation whereas, on the other hand, the adhesion must not prevent a rapid and simple stripping during mounting. The outer conducting layer is applied by extrusion around the insulation. The conducting polymer composition preferably consists of mixtures comprising a copolymer of ethylene and a second polar monomer such as vinyl acetate, ethylacrylate, butylacrylate, methylacrylate, etc., and a polyolefin, pre¬ ferably a crystalline polypropylene. The mixture may also contain an elastomer such as a copolymer or terpolymer of ethylene propylene. This polymer composition also comprises an electrically conducting filler such as carbon black to impart to the polymer compound the necessary electrical conductivity and also a stabilization system comprising, inter alia, antioxidants to counteract degradation as a result of external influence such as oxidation, radiation and electric load. Where there is a need to crosslink the polymer composition, additives of any agents necessary for crosslinking of the polymer composition may also be included. The strippability of the composition layer depends on its adhesion to the underlying insulating material in combination with its mechanical properties. Above all in connection with cables comprising an inner insulating material consisting of a polyethylene crosslinked by means of silane technique, the strippability of the outer conducting layer, is, however, for known conducting polymer compositions deteriorated with increasing temperature and even at temperatures around 30°C problems arise as a consequence of the physical properties of the polymer composition changing with the temperature. For an electrically conducting polymer composition intended for use in a cable insulation as external strippable layer around an insulation of a thermoplastic or crosslinkable polymer composition, for example polyethylene, it is known to use the following compositions: -20 to 70 per cent by weight of an ethylene vinyl acetate copolymer, EVA, consisting of from 20 to 80 per cent by weight ethylene and 80 to 20 per cent by weight vinyl acetate, - 5 to 60 per cent by weight polypropylene, PP, - up to 30 per cent by weight of an ethylene propylene copolymer, EPM, or a terpolymer of ethylene propylene, EPDM, - 5 to 60 per cent by weight of an electrically conducting filler such as carbon black, and - up to 5 per cent by weight of stabilizing additives which at least comprise one antioxidant. It is known that in a polymer composition of the above type, which contains a copolymer in the form of ethylene vinyl acetate, EVA, the temperature dependence of the mechanical properties may be reduced by, for example, increasing the ethylene content in the mixture, which can be obtained if the content of vinyl acetate in the copolymer is reduced. However, the thus increased ethylene content in both the copolymer and the polymer composition leads to an unwanted increase of the adhesion of the polymer composition to the polyethylene insu¬ lation. So in spite of the temperature dependence of the mechanical properties, no improvement of the strippability is obtained at temperatures above room temperature when the adhesion of the polymer composition to the polyethylene insu¬ lation increases. It is an object of the invention to suggest an electrically conducting polymer composition which has a thermally stable adhesion and where the adhesion is optimized to be suffi¬ ciently good to ensure that the outer conducting layer makes contact with the insulation such that no corona arises between the insulating material and the conducting layer at normal electric load. The adhesion to the insulating material is not higher than that the conducting layer can be easily removed, be stripped, from the polymeric insulating material within a wide temperature interval corresponding to a majority of con- ceivable conditions during use and mounting, preferably within a temperature interval of -20°C to +65°C. SUMMARY OF THE INVENTION An extrudable electrically conducting polymer composition in the form of a mixture comprising at least: -20 to 70 per cent by weight of a copolymer consisting of 20 to 80 per cent of ethylene and of 80 to 20 per cent by weight of a polar monomer, such as vinyl acetate, ethylacrylate, methylacrylate or butylacrylate, - 5 to 60 per cent by weight of a polypropylene, - 5 to 60 per cent by weight of an electrically conducting filler such as carbon black, to give the desired conductivity, and - up to 5 per cent by weight of stabilizing additives such as antioxidants, is given a greater thermal stability of the mechanical proper- ties, especially at temperatures above room temperature, by modifying the polarity of the polypropylene according to the invention. The modified polypropylene used according to the invention is a graft copolymer of a crystalline polypropylene and a carbo- xylic acid anhydride, or a block copolymer of a crystalline polypropylene and a co- or terpolymer of ethylene propylene. In one embodiment of the invention the above-mentioned polymer composition is crosslinkable by means of silane technique. The polymer composition according to this embodiment has been reacted with a silane containing hydrolyzable groups, whereby the polymer composition can be crosslinked by bringing the formed product into contact with moisture or water such that hydrolyzable groups in the silane compound are hydrolyzed and, by condensation, give rise to siloxane bonds between the grafted molecules in the polymer composition. Silane grafting is described in greater detail in the following patent speci¬ fications: US 3 075 948 and US 3 646 155. With a polymer composition according to the invention, an extrudable electrically conducting polymer composition is obtained which, in relation to known polymer compositions, for the described application, as an outer conducting layer in an insulating system for electric cables, exhibits a greater thermal stability of the mechanical properties. In addition, an outer conducting layer of a polymer composition according to the invention, which by extrusion has been applied to a polyethylene insulation included in an electric cable, exhibits good strippability within a great temperature inter¬ val, preferably between -20°C and +65°C. The conducting layer can be easily stripped from an insulating layer since the adhesion of a polymer composition layer, according to the invention, to a polyethylene insulation is not higher than what is required to ensure that corona does not occur between the layer and the insulation. In one embodiment of the invention, the extrudable electri¬ cally conducting polymer composition comprises, in addition to carbon black, any fillers and the necessary stabilization system, at least one copolymer of ethylene and vinyl acetate, EVA, mixed with polypropylene, PP, where according to the invention the polypropylene has been modified by polar groups and where the content of vinyl acetate in the ethylene vinyl acetate copolymer has been reduced, thus obtaining a polymer composition which, within the temperature interval described above, exhibits a stable adhesion to the polyethylene insu- lation. In addition to the desired greater thermal stability of the mechanical properties of a polymer composition according to the invention, it also exhibits, as a result of the fact that the polyolefin included is modified, a plurality of advantages from the point of view of process technique in relation to known polymer compositions of this type. The mixing process is simplified since during the mixing of a polymer composition according to the invention only components which are compatible with each other are supplied to the mixer, and, in addition, the modification of the polyolefin improves its wetting ability towards carbon black. The invention will be described in greater detail in the following with reference to a number of examples. EXAMPLE 1 An extrudable electrically conducting polymer composition A, XG 9607:70 from Neste Polymer Compounds AB. The material is a thermoplastic composition which is used as strippable outer layer for medium voltage cables with silane crosslinked polyethylene insulation. From the polymer composition A, test bars were produced for tensile testing at room temperature, RT, and 50°C in a tensile testing machine. In addition, the polymer composition was extruded as an outer casing over a cable insulation of silane- crosslinked polyethylene. The strippability was determined in a tensile testing machine at a tensile angle of 180°, a ten¬ sile speed of 100 mm/min. and the following temperatures: -10°C, RT, +40°C, +45°C, +50°C and +60°C. The results from tensile testing and strippability testing are shown in table 1. EXAMPLE 2 An extrudable electrically conducting polymer composition B was prepared by compounding the following components in a double-screw extruder into a homogeneous compound: 100 parts by weight of an ethylene vinyl acetate copolymer, EVA, containing 72 per cent by weight ethylene and 28 per cent by weight vinyl acetate, 42 parts by weight polypropylene, PP, 25 parts by weight of a terpolymer of ethylene propylene, EPDM, 62 parts by weight carbon black of conducting type, and 5.6 parts by weight of a stabilization system comprising at least one antioxidant. From the polymer composition obtained, test bars were manufactured for tensile testing at room temperature, RT, and 50°C in a tensile testing machine. In addition, the polymer composition was extruded as an outer casing over the cable insulation of silane-crosslinked polyethylene. The results are shown in Table 1. EXAMPLE 3 An extrudable electrically conducting polymer composition C with a composition according to the invention was prepared by compounding the following components in a double-screw extruder into a homogeneous compound: 100 parts by weight of an ethylene vinyl acetate copolymer, EVA, containing 72 per cent by weight ethylene and 28 per cent by weight vinyl acetate, 40 parts by weight modified polypropylene, PP carboxylic acid anhydride, 46 parts by weight carbon black of conducting type, and 5.6 parts by weight of a stabilization system comprising a least one antioxidant. From the polymer composition obtained, test bars were manufactured for tensile testing at room temperature, RT, and 50°C in a tensile testing machine. In addition, the polymer composition was extruded as an outer casing over a cable insulation of silane-crosslinked polyethylene. The strippa¬ bility was determined in a tensile testing machine at a ten¬ sile angle of 180°, a tensile speed of 100 mm/min and the following temperatures: -10°C, RT, +40°C, +45°C, +50°C and +60°C. The results are shown in Table 1. EXAMPLE 4 An extrudable electrically conducting polymer composition D with a composition according to the invention was prepared by compounding the following components in a double-screw extruder into a homogeneous compound: 100 parts by weight of an ethylene vinyl acetate copolymer, EVA, containing 72 per cent by weight ethylene and 28 per cent by weight vinyl acetate, 70 parts by weight modified polypropylene in the form of a copolymer of polypropylene and EPDM, 62 parts by weight carbon black of conducting type, and 5.6 parts by weight of a stabilization system comprising a least one antioxidant. From the polymer composition obtained, test bars were manufactured for tensile testing at room temperature, RT, and 50°C in a tensile testing machine. In addition, the polymer composition was extruded as an outer casing over a cable insulation of silane-crosslinked polyethylene. The strippa- bility was determined in a tensile testing machine at a ten¬ sile angle of 180°, a tensile speed of 100 mm/min and the following temperatures: -10°C, RT, +40°C, +45°C, +50°C and +60°C. The results are shown in Table 1. Table 1 Composition A B Tensile testing RT σ(MPa) 10 15.2 13.4 12.8 ε(%) 420 265 260 290 50°c σ(MPa) 3.0 6.2 5.8 5.2 ε(%) 150 230 220 280 Adhesion (N/cm, should amount to between 10 and 20 N/cm) -10°C * >35 >15 >15 RT 18 >35 14 16 +40°C # >35 14 10 +45°C # >35 14 10 +50°C # >30 13 10 +60°C # >30 13 10 * strippable but adhesion not measured # not strippable

Claims: CLAIMS 1. An extrudable electrically conducting polymer composition in the form of a mixture comprising at least: - 20 to 70 per cent by weight of a copolymer which consists of 20 to 80 per cent by weight of ethylene and 80 to 20 per cent by weight of a polar monomer, such as vinyl acetate, ethylene acrylate, methylacrylate, butylacrylate, - 5 to 60 per cent by weight of a polyolefin, - 5 to 60 per cent by weight of an electrically conducting filler such as carbon black, to provide the desired electrical conductivity, and - up to 5 per cent by weight of stabilizing additives such as antioxidants, characterized in said polyolefin is a polypropylene with modified polarity. 2. A polymer composition according to claim 1, characterized in that said modified polyolefin comprises polar groups which, in the form of a carboxylic acid or its anhydride, by a graft reaction has been introduced into the polyolefin. 3. A polymer composition according to claim 1, characterized in that said modified polyolefin is a block copolymer between polypropylene and ethylene propylene rubber. 4. A polymer composition according to any of the preceding claims, characterized in that said composition comprises a silane with hydrolyzable groups intended to be crosslinked by hydrolysis in case of water or moisture absorption. 5. An insulated electric cable with an insulating system which at least comprises an electrically insulating layer of polyethylene, arranged around an electric conductor, and a layer of an electrically conducting polymer composition arranged around said insulation, characterized in that said outer conducting layer is a polymer composition according to any of the preceding claims, applied to a thermoplastic or crosslinkable polyethylene insulation by extrusion. 6. An insulated electric cable according to claim 5, characterized in that said outer conducting layer is strippable from the polyethylene insulation with a stripping force of less than 20 Ncirr---.

Inventors: RYPINSKA OLSSON MARIA, RYPINSKA OLSSON, MARIA
Applicants: ASEA BROWN BOVERI, RYPINSKA OLSSON MARIA, ASEA BROWN BOVERI AB, RYPINSKA OLSSON, MARIA
Application year: 1994
Main classification: C08L23
IPC codes: C08L23, C08L51, H01B1